gallivm: use f16c hw support for float->half and half->float conversion

gallivm: use f16c hw support for float->half and half->float conversion

Should be way faster of course on cpus supporting this (includes AMD
Bulldozer and Jaguar cores, Intel Ivy Bridge and up (except budget models)).
Passes piglit fbo-blending-formats GL_ARB_texture_float -auto on Ivy Bridge.

Reviewed-by: Brian Paul <brianp@vmware.com>